Bug 14016: Restore correct date format on issue date (00:00 vs 23:59)

Bug 14016: Restore correct date format on issue date (00:00 vs 23:59)

Introduced by bug 13601, and same fix used in bug 10423 and bug 12847:
the date_due retrieved from the DB is modified.

There are some problems:
1/ There is confusion between the iso and sql formats in the codebase.
2/ Since bug 13601, dt_from_string does not manage the iso format (there
are occurrences of 'iso' but it assumes that both formats are
identical).

To solve the issue, 2 solutions:
1/ Same as bug 10423 and bug 12847: try to get rid of the change done on
date_due in C4::Members::GetPendingIssues, it should be kept as the sql
value.
2/ Too many errors found and another fallback should be added to
dt_from_string (if 'iso' is passed, try sql then iso).

Test plan:
Go on the checkout list at the OPAC and confirm that the due dates are
correctly formatted.

Signed-off-by: Nicolas Legrand <nicolas.legrand@bulac.fr>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>